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        <p>对于传统的 JavaScript 程序，一般使用<strong>函数</strong>和<strong>基于原型的继承</strong>来创建可重用的组件，但是面向对象的语言用的是<strong>基于类的继承</strong>，并且对象是由类构建出来的。 </p>
<p>从 ES6 开始， JavaScript 能够使用基于类的面向对象的方式。 使用 TypeScript，允许开发者现在就使用这些特性，并且编译后的 JavaScript 可以在所有主流浏览器和平台上运行，而不需要等到下个 JavaScript 版本。</p>
<h3 id="1-示例"><a href="#1-示例" class="headerlink" title="1. 示例"></a>1. 示例</h3><figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Person</span> &#123;</span><br><span class="line">    <span class="comment">// 成员变量</span></span><br><span class="line">    <span class="attr">name</span>: <span class="built_in">string</span>;</span><br><span class="line">    <span class="attr">age</span>: <span class="built_in">number</span>;</span><br><span class="line">    <span class="attr">gender</span>: <span class="built_in">string</span>;</span><br><span class="line">    <span class="comment">// 构造方法</span></span><br><span class="line">    <span class="title function_">constructor</span>(<span class="params">name: <span class="built_in">string</span>, age: <span class="built_in">number</span>, gender: <span class="built_in">string</span></span>) &#123;</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">name</span> = name;</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">age</span> = age;</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">gender</span> = gender;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">// 成员方法</span></span><br><span class="line">    <span class="title function_">sayHi</span>(): <span class="built_in">string</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="string">`I&#x27;m <span class="subst">$&#123;<span class="variable language_">this</span>.name&#125;</span>, a <span class="subst">$&#123;<span class="variable language_">this</span>.age&#125;</span> years old <span class="subst">$&#123;<span class="variable language_">this</span>.gender&#125;</span>.`</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// 创建类的实例</span></span><br><span class="line"><span class="keyword">const</span> person = <span class="keyword">new</span> <span class="title class_">Person</span>(<span class="string">&#x27;Horsen&#x27;</span>, <span class="number">3</span>, <span class="string">&#x27;boy&#x27;</span>);</span><br><span class="line"><span class="comment">// 调用实例的方法</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(person.<span class="title function_">sayHi</span>());</span><br></pre></td></tr></table></figure>

<p>这种写法在 Java 程序员看来十分舒适。</p>
<span id="more"></span>

<h3 id="2-继承"><a href="#2-继承" class="headerlink" title="2. 继承"></a>2. 继承</h3><p>说到继承，就不得不提到多态。看到 TypeScript 可以这样写，作为 Java 程序员我感到十分欣慰：</p>
<figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Student</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Person</span> &#123;</span><br><span class="line">    <span class="comment">// 新加的成员变量</span></span><br><span class="line">    <span class="attr">school</span>: <span class="built_in">string</span>;</span><br><span class="line">    <span class="comment">// 构造方法</span></span><br><span class="line">    <span class="title function_">constructor</span>(<span class="params">name: <span class="built_in">string</span>, age: <span class="built_in">number</span>, gender: <span class="built_in">string</span>, school: <span class="built_in">string</span></span>) &#123;</span><br><span class="line">        <span class="comment">// 调用父类的构造方法</span></span><br><span class="line">        <span class="variable language_">super</span>(name, age, gender);</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">school</span> = school;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">// 重写父类的方法</span></span><br><span class="line">    <span class="title function_">sayHi</span>(): <span class="built_in">string</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="string">`I&#x27;m <span class="subst">$&#123;<span class="variable language_">this</span>.name&#125;</span>, a <span class="subst">$&#123;<span class="variable language_">this</span>.age&#125;</span> years old <span class="subst">$&#123;<span class="variable language_">this</span>.gender&#125;</span> who graduated from <span class="subst">$&#123;<span class="variable language_">this</span>.school&#125;</span>.`</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// 父类引用指向子类实例</span></span><br><span class="line"><span class="keyword">const</span> <span class="attr">student</span>:<span class="title class_">Person</span> = <span class="keyword">new</span> <span class="title class_">Student</span>(<span class="string">&#x27;Horsen&#x27;</span>, <span class="number">3</span>, <span class="string">&#x27;boy&#x27;</span>, <span class="string">&#x27;NJIT&#x27;</span>);</span><br><span class="line"><span class="comment">// 这里调用的是子类的sayHi方法, 这属于运行时多态</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(student.<span class="title function_">sayHi</span>());</span><br></pre></td></tr></table></figure>

<p>当然子类引用也可以指向父类实例，前提是子类不能又扩展的方法，但是这样的话子类就没有意义了，所以一般不会这么写。</p>
<h3 id="3-访问限定符"><a href="#3-访问限定符" class="headerlink" title="3. 访问限定符"></a>3. 访问限定符</h3><p>和 Java 一样。</p>
<table>
<thead>
<tr>
<th>访问修饰符</th>
<th>本类</th>
<th>子类</th>
<th>其他类</th>
</tr>
</thead>
<tbody><tr>
<td>public</td>
<td>✅</td>
<td>✅</td>
<td>✅</td>
</tr>
<tr>
<td>protected</td>
<td>✅</td>
<td>✅</td>
<td>❌</td>
</tr>
<tr>
<td>private</td>
<td>✅</td>
<td>❌</td>
<td>❌</td>
</tr>
</tbody></table>
<h3 id="4-readonly-修饰符"><a href="#4-readonly-修饰符" class="headerlink" title="4. readonly 修饰符"></a>4. readonly 修饰符</h3><p>使用<code>readonly</code>关键字将属性设置为只读的，既然是只读的，那就必须在声明时或构造函数里被初始化，否则就没有声明它的意义：</p>
<figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Person</span> &#123;</span><br><span class="line">    <span class="keyword">readonly</span> <span class="attr">name</span>: <span class="built_in">string</span> = <span class="string">&#x27;Horsen&#x27;</span>;</span><br><span class="line">    <span class="title function_">constructor</span>(<span class="params">name: <span class="built_in">string</span></span>) &#123;</span><br><span class="line">      <span class="variable language_">this</span>.<span class="property">name</span> = name;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">let</span> person = <span class="keyword">new</span> <span class="title class_">Person</span>(<span class="string">&#x27;HorsenLiu&#x27;</span>);</span><br></pre></td></tr></table></figure>

<p>视频看到这里的时候弹幕提了一个问题：</p>
<blockquote>
<p>你不能用 private 修饰吗？</p>
</blockquote>
<p>不能。private 的作用是限制属性只在类内部可读可写，在类外不可读不可写；然而 readonly 允许属性在类外可读不可写，并且在类内部也是可读不可写。</p>
<p>利用<strong>参数属性</strong>可以更方便的做到上面的事情：</p>
<figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Person</span> &#123;</span><br><span class="line">    <span class="title function_">constructor</span>(<span class="params"><span class="keyword">readonly</span> name: <span class="built_in">string</span></span>) &#123;&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">let</span> person = <span class="keyword">new</span> <span class="title class_">Person</span>(<span class="string">&#x27;HorsenLiu&#x27;</span>);</span><br></pre></td></tr></table></figure>

<p>同样的，使用 private、public 或 protected 修饰一个参数属性时，也会声明并初始化一个私有成员。</p>
<h3 id="5-getter-x2F-setter"><a href="#5-getter-x2F-setter" class="headerlink" title="5. getter&#x2F;setter"></a>5. getter&#x2F;setter</h3><p>和 Java 中的 getter&#x2F;setter 在功能上是一样的，但是写法和用法上有一些区别：</p>
<figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Teacher</span> &#123;</span><br><span class="line">    <span class="attr">name</span>: <span class="built_in">string</span>;</span><br><span class="line">    <span class="keyword">get</span> <span class="title function_">getName</span>() &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="variable language_">this</span>.<span class="property">name</span>;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">set</span> <span class="title function_">setName</span>(<span class="params">name: <span class="built_in">string</span></span>) &#123;</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">name</span> = name;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">const</span> teacher = <span class="keyword">new</span> <span class="title class_">Teacher</span>();</span><br><span class="line">teacher.<span class="property">setName</span> = <span class="string">&#x27;Liu&#x27;</span>;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(teacher.<span class="property">getName</span>); <span class="comment">// Liu</span></span><br></pre></td></tr></table></figure>

<p><em>注意：这里的 getter 和 setter 不应当理解成方法，比起 Java 中直接调用的 getter&#x2F;setter 方法，这更像是负责某一功能的一个属性。</em></p>
<h3 id="6-静态属性"><a href="#6-静态属性" class="headerlink" title="6. 静态属性"></a>6. 静态属性</h3><figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Counter</span>&#123;</span><br><span class="line">    <span class="keyword">static</span> <span class="attr">counter</span>:<span class="built_in">number</span> = <span class="number">0</span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="title class_">Counter</span>.<span class="property">counter</span>++;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="title class_">Counter</span>.<span class="property">counter</span>); <span class="comment">// 1</span></span><br></pre></td></tr></table></figure>

<p>和 Java 中的用法一样。</p>
<h3 id="7-抽象类"><a href="#7-抽象类" class="headerlink" title="7. 抽象类"></a>7. 抽象类</h3><p>抽象类作为其他子类的基类，不能被实例化。不同于接口，抽象类可以包含成员的实现细节：</p>
<figure class="highlight ts"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">abstract</span> <span class="keyword">class</span> <span class="title class_">Animal</span> &#123;</span><br><span class="line">    <span class="attr">name</span>: <span class="built_in">string</span>;</span><br><span class="line">    <span class="keyword">abstract</span> <span class="title function_">shout</span>();</span><br><span class="line">    <span class="title function_">run</span>(<span class="params"></span>) &#123;</span><br><span class="line">        <span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="string">`<span class="subst">$&#123;<span class="variable language_">this</span>.name&#125;</span> is running!`</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Dog</span> <span class="keyword">extends</span> <span class="title class_ inherited__">Animal</span> &#123;</span><br><span class="line">    <span class="title function_">shout</span>(<span class="params"></span>) &#123;</span><br><span class="line">        <span class="variable language_">this</span>.<span class="property">name</span> = <span class="string">&#x27;Dog&#x27;</span>;</span><br><span class="line">        <span class="variable language_">console</span>.<span class="title function_">log</span>(<span class="string">`<span class="subst">$&#123;<span class="variable language_">this</span>.name&#125;</span> is shouting!`</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">const</span> dog = <span class="keyword">new</span> <span class="title class_">Dog</span>();</span><br><span class="line">dog.<span class="title function_">shout</span>();</span><br><span class="line">dog.<span class="title function_">run</span>();</span><br></pre></td></tr></table></figure>


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